Impact devices are used for breaking or drilling concrete, tamping soils, riveting, and other such difficult tasks. For many years such devices have been powered by compressed air. More recently hydraulic powdered devices, and devices utilizing electrical power driving a cylinder-piston air spring, a coil spring, or other resilient drive element have been devised which have comparable capability.
Although such prior art devices develop high impacting capability in the ram, they fail to transmit and apply such high ram impacting capability through the tool to the work to effectively and efficiently accomplish the work intended.
All such prior art devices incorporate power drive means to drive a ram into reciprocating impacting motion by action through a resilient drive element supported directly or indirectly against the frame. Accordingly such power drive means also actuate the frame into opposite reactive motion. Such reactive motion is the principal cause of the deficiencies of such prior art devices, in applying such high performance impacting of the ram to accomplish the work inteneded.
Other, less effective, prior art means for actuating a ram into impacting motion include rotatiing eccentric weights pivoted directly on the ram. Such devices (1) are limited to impacting accelerations low enough to maintain force loadings on the pivots and rotary drive means within practical bounds, (2) require heavy rotating weights, not necessary with other prior art devices, and (3) have problems of phasing the rotations of the weights with the ram reciprocations to obtain reasonable impacting effectiveness. U.S. Pat. No. 2,176,801 (Oct. 17, 1939) discloses an embodiment incorporating such rotating weights with an air spring and two coil springs to provide such phasing. Another prior art means for actuating the ram, as disclosed in U.S. Pat. No. 973,216 (Oct. 18, 1910), incorporates a magnetic coupling means, which is heavy, ineffective, and requires an auxiliary electric power source. Because of such deficiencies, impact devices incorporating rotating eccentrics or magnetic means have had no known practical acceptance.
In all such devices each impact is generated by the sudden stopping of a moving ram. At least two elements are involved in such action; a ram and a tool. These elements may be (1) secured together, as the head and point of a pickax, which are thrust, as a unit, toward a workpiece, as disclosed in U.S. Pat. No. 2,420,793 (May 20, 1947); or, (2) the tool may be held separately against the workpiece to receive impact of the ram and transmit the resulting force sharply to the workpiece, as in a hammer-chisel action. For most applications, especially for working harder surfaces, years of experience in the prior art have shown the hammer-chiesel action to be much more effective than pickax action.
Typically, for high performance piror art impact devices, the ram impacts a tool positioned by a tool holder on the frame to accept such impacting, which is transmitted from ram to tool to accomplish the work, ideally by hammer-chisel action as described hereinbefore. In all known prior art devices, however, an ideal hammer-chisel action is not attained.
The output means of most prior art impact devices have motion-limiting stops on the tool and matching stops on the tool holder to limit the maximum travel of the tool in the tool holder, and a tool retainer, which also serves as one of the stops, to prevent accidental expulsion of an unsupported tool. In many prior art devices, particularly larger units, additionally an anvil is interposed between ram and tool. An embodiment of such output means is schematically shown in FIG. 2. When the impact device is not operating and frame 12 is biased downward by an operator, frame 12 forces ledge 79 against anvil 57 which in turn presses tool 62 against workpiece 84.
However, when in operation, especially for devices in which the sharp down acceleration of ram 44 is obtained by reaction between ram 44 and frame 12, stop means 79 moves away from anvil 57. Thus, immediately prior to each impact no bias force holds the anvil to the tool nor the tool to the workpiece. Furthermore, friction between tool and tool holder urges the anvil clear of the tool and the tool clear of the workpiece. In addition, under strong impacting, especially on harder materials, the workpiece deflects and vibrates as an elastic body, and as a whole with respect to its supporting means. Such vibrations, plus the release of compression occurring in the tool and anvil caused by the impact, reflect back on tool and anvil and cause each to rebound. FIG. 2-B illustrates a typical position of frame, anvil, and tool relative to a workpiece just prior to impact of the ram as a result of such factors of frame reaction, friction and rebound.
Prior art devices have no means for eliminating such tool and anvil spurious movements, other than relying upon the operator applying sufficient bias force. For prior devices utilizing high performance power drive means with a resilient drive element as described hereinbefore, it is difficult or impossible with any reasonable bias force from the operator to hold stop means 79 against anvil 57 and thereby tool 62 firmly against workpiece 84.
The loss of efficiency and effectiveness from such spurious motions is serious. When an unsupported anvil or tool, is impacted either at rest pr in motion, only part of the ram kinetic energy and momentum is transferred. As much as 10 to 25% is dissipated or lost by vibrations and heating in such collisions. Also, the tool being cast toward the workpiece, the output means no longer acts by the hammer-chisel principle but by the less effective pickax principle, with only the mass of the tool, without the ram, striking the workpiece, resulting in further loss of effectiveness.
FIGS. 2-C, 2-D, and 2-E show a sequence of typical unsupported collisions of ram to anvil, anvil to tool, and tool to workpiece respectively. At each of these respective collisions the energy and momentum of impact is degraded as described hereinbefore.
In many smaller prior art impact devices, no anvil is incorporated, the ram impacting the tool directly. Referring to FIG. 2-A, with anvil 57 omitted, as bias force is applied to frame 12 when the device is inoperative, ledge 86 presses tool collar 70 and hence tool 62 to the workpiece. For such devices, when in operation, ledge 86 moves away from collar 70 and except for no losses because of the absence of any anvil, the problems and deficiencies described hereinbefore are the same.
In addition to the aforementioned spurious tool motions, the frame, which has been forced away from contact with anvil or tool just prior to each impact as shown in FIG. 2-B, is then forced rapidly back to impact anvil 57, for example, as the ram is moved upward after each impact as shown in FIG. 2-A.
In a rapid succession of such direct frame impacts, the frame dynamic motion is highly non-linear and irregular, and the frame rebounds. This increases further the difficulties of holding the frame to the tool. If large enough bias force is applied to hold the frame rigidly against the tool just prior to contact, when the ram mass moves in a direction away from the workpiece, the center of mass of the entire impact device also moves away. Accordingly, starting from this higher position of center of mass, the tendency for the frame to move away from direct support of the anvil or tool is greater and the problem is increased.
While the foregoing description is illustrative of the dynamic situation occuring in typical state-of-art devices, the actual dynamics are much more complex and result in considerable net loss in energy and momentum and hence effectiveness in the output means of state-of-art devices incorporating a resilient drive element, and particularly a mechanical resilient drive element, in the power drive means.
As a partial solution of such problems in some prior devices, additional masses, including a cylinder reciprocated by crank-connecting rod means, are reciprocated with respect to the frame and in opposition to the ram reciprocating motion, to reduce the frame reaction motion, somewhat as in commonly used vibration dampers. Also, in some prior devices longer time periods between impacts is incorporated to improve the aforementioned conditions. Such partial improvments are heavy or can only be obtained at much lower impact rates, as low as half the commonly used rate, and accordingly restrict performance and do nothing significant to correct the initial removal of bias force from the anvil and/or tool caused by the sharp reaction motion of the frame immediately before impact.
In some prior art impact devices additional means are incorporated in the output means to accomplish certain results. Most common of such means are resilient relife means incorporated, especially on larger high impact devices. Such relief means usually include a slidable tool holder and relief coil springs, arranged to hold the tool holder against the frame during normal operation, and to relieve peak forces during operation with no tool or with the tool free of a workpiece.
Another arrangement, incorporated with output means on an electromechanical impact device disclosed in U.S. Pat. No. 973,216 (Oct. 18, 1910), is a relief spring arranged to hold the tool when idle, in a position to avoid ram impacting. To receive the ram impacting, the tool is pressed into position by a workpiece, which compresses the spring against the frame. Accordingly, this arrangement restricts frame motion and has the accompanying deficiencies discussed hereinbefore.
U.S. Pat. No. 1,107,550 (Aug. 18, 1914) shows another spring incorporated in a tool holder as part of an on-off air valve. The impact device operates when the tool is held against a workpiece and the spring is compressed to a limiting stop to correctly locate the tool holder adjacent a duct opening. This restriction does not permit free reactive motion of the frame during operation and accordingly, has the aforementioned deficiencies.
Because of the deficiencies described hereinbefore, an improved device is much sought after. Thus, it would be advantageous to have a more efficient and effective device in which the high impacting capability of a ram is directed more toward accomplishing work and less to unnecessary random and spurious motions and collisions of ram, anvil, tool, and frame.